The World Will Be More Crowded — With Old People

Demography is not destiny, as is sometimes claimed. The human race could be wiped out by a plague or an asteroid, or transformed by some new technology. But no matter what, today's patterns of fertility, migration, and mortality fundamentally determine how much society will or can change for many generations to come.

And what demography tells us is this: The human population will continue to grow, though in a very different way from in the past. The United Nations' most recent "mid-range" projection calls for an increase to 8 billion people by 2025 and to 10.1 billion by century's end.

Until quite recently, such population growth always came primarily from increases in the numbers of young people. Between 1950 and 1990, for example, increases in the number of people under 30 accounted for more than half of the growth of the world's population, while only 12 percent came from increases in the ranks of those over 60.

But in the future it will be the exact opposite. The U.N. now projects that over the next 40 years, more than half (58 percent) of the world's population growth will come from increases in the number of people over 60, while only 6 percent will come from people under 30. Indeed, the U.N. projects that by 2025, the population of children under 5, already in steep decline in most developed countries, will be falling globally -- and that's even after assuming a substantial rebound in birth rates in the developing world. A gray tsunami will be sweeping the planet.

Which countries will be aging most rapidly in 2025? They won't be in Europe, where birth rates fell comparatively gradually and now show some signs of ticking up. Instead, they'll be places like Iran and Mexico, which experienced youth bulges that were followed quickly by a collapse in birth rates. In just 35 years, both Iran and Mexico will have a larger percentage of their populations over 60 than France does today. Other places with birth rates now below replacement levels include not just old Europe but also developing countries such as Brazil, Chile, China, Lebanon, Tunisia, South Korea, and Vietnam.

Because of the phenomenon of hyper-aging in the developing world, another great variable is already changing as well: migration. In Mexico, for example, the population of children age 4 and under was 434,000 less in 2010 than it was in 1996. The result? The demographic momentum that fueled huge flows of Mexican migration to the United States has waned, and will wane much more in the future. Already, the net flow of illegal Mexican immigration northward has slowed to a trickle. With fewer children to support and not yet burdened by a huge surge of elders, the Mexican economy is doing much better than in the past, giving people less reason to leave. By 2025, young people on both sides of the border may struggle to understand why their parents' generation built this huge fence.

Despite these trends, most people conclude from their day-to-day lives that overpopulation is a serious problem. One reason is that more than half the world's population is crowded into urban areas. The high cost of raising children in mega-cities is a prime reason that global birth rates continue to fall, yet urbanization also makes the larger trend toward depopulation difficult for most to grasp. If the downward trend in birth rates doesn't moderate and stabilize as the U.N. assumes it will, the world as a whole could be losing population as soon as midcentury. And yet few people will likely see that turning point coming, so long as humans continue to pack into urban areas and increase their consumption of just about everything.

Another related megatrend is the rapid change in the size, structure, and nature of the family. In many countries, such as Germany, Japan, Russia, and South Korea, the one-child family is now becoming the norm. This trend creates a society in which not only do most people have no siblings, but also no aunts, uncles, cousins, nieces, or nephews. Many will lack children of their own as well. Today about one in five people in advanced Western countries, including the United States, remains childless. Huge portions of the world's population will thus have no biological relatives except their parents.

And even where children continue to be born, they are being raised under radically different circumstances, as country after country has seen divorce and out-of-wedlock births surge and the percentage of children living with both of their married parents drop sharply. So not only is the quantity of children in the world poised to shrink rapidly, but on current trends, a near majority of them will be raised in ways that are today strongly associated with negative life outcomes.

Are there signs of any of these trends reversing before 2025? Only a few. The percentage of the world's population raised in religious households is bound to rise, if only because adherents to fundamentalism, whether Christians, Jews, or Muslims, tend to have substantially more children than their secular counterparts. And there are certainly many ways -- from increased automation and delayed retirement to health-care reform to the provision of baby bonuses -- for societies to at least partially adjust to the tidal shift in global demographics.

But don't count on it. To make such sweeping changes would require a widespread understanding of the century's great paradox: The planet may be bursting, but most of this new population is made up of people who have already been born. So get ready for a planet that's a whole lot more crowded -- with old people.